1. Field of the Invention
The present invention relates to a two-stage reaction device for vacuum brake boosters with a vacuum chamber in which a constant pressure prevails and with a working chamber in which varying pressures prevail, the chambers being separated by a movable wall. A control valve actuatable by a brake pedal governs the differentials of pressure acting on the movable wall. A push rod controlled by the brake pedal acts on a master cylinder and on levers which are in engagement with the movable wall at a first location. The push rod is connected to a plate at a second location, the said first location being arranged radially farther outwardly than the second location. The levers are acted upon by a cup spring at a third radially spaced location, the cup spring being arranged on the side of the levers close to the vacuum chamber. The third location is located further outwardly radially than the first location. A spring plate is interposed between the cup spring and the levers which is provided with bearings for the levers at the side close to said levers.
2. Description of the Prior Art
From U.S. Pat. No. 3,102,453, issued Sept. 3, 1963 to F. W. Brooks et al., a vacuum brake booster is known which comprises a constant atmospheric pressure chamber and a working chamber in which different pressures prevail, as well as a movable wall dividing the chambers, the wall being mounted on a push rod, and a control valve which is actuatable by a brake pedal. The control valve controls the differentials of pressure acting on the movable wall. The end of the push rod adjacent to the control valve is provided with a pin having a plate pressed thereon. Disposed between the plate and the control valve are three reaction levers which, on the one side, take support radially outwardly against the movable wall and radially inwardly against a spring, while they bear against the plate on the other side. In this arrangement, a spring is located in an opening in the valve piston of the control valve, the opening being adjacent to the vacuum chamber. The spring acts to decouple the piston rod from the push rod. The bias of the spring provides for what is termed "two-stage action" which means the retardation of the reaction force acting on the brake pedal. The magnitude of the "two-stage action" is dependent upon the strength of the reaction-delaying spring and the distances between the points of force impact.
In known vacuum brake boosters with two-stage reaction devices, the reaction levers (such as those shown in FIG. 3 of the cited Brooks et al. reference) bear with their outer ends directly against a component which directs the control valve and which is rigidly connected to the movable wall so that, as explained previously, the reaction levers act on the movable wall. This lever assembly cannot be utilized in devices comprising a control housing made of any thermoplastics, since the latter material does not permit the levers to bear directly against the control housing. Moreover, the space available in this known arrangement is so small that assembly problems frequently arise.